A magnetic tunnel junction (MTJ) element as a magnetoresistive element has a basic structure that is a stack structure that includes a storage layer having a changeable magnetization direction, a reference layer having a fixed magnetization direction, and an insulating layer disposed between the storage layer and the reference layer. This MTJ element is known to have a tunneling magnetoresistive (TMR) effect, and is used as the storage element of a memory cell in a magnetoresistive random access memory (MRAM).
An MRAM stores information (“1” or “0”) depending on changes in the relative angle between the magnetization directions of the magnetic layers in each MTJ element, and is nonvolatile. As the magnetization switching speed is several nanoseconds, high-speed data writing and high-speed data reading can be performed. In view of this, MRAMs are expected to be next-generation high-speed nonvolatile memories. Further, where a technique called spin transfer torque switching is used to control magnetization with a spin-polarized current, the cell size in an MRAM is reduced, and the current density increases accordingly. Thus, the magnetization of the storage layer can be readily reversed, and a high-density MRAM that consumes less power can be formed.
To increase the density of a nonvolatile memory, a higher degree of magnetoresistive element integration is essential. Also, to lower the write current while achieving a higher degree of integration, MRAMs using MTJ elements containing magnetic materials that have magnetizations perpendicular to the film surfaces have been developed in recent years.
Normally, an inversion current for reversing a magnetization by the spin transfer torque switching technique depends on the saturation magnetization and the magnetic relaxation constant of the storage layer. Therefore, there is a demand for a storage layer that has a lower saturation magnetization and a lower magnetic relaxation constant so that the magnetization of the storage layer can be reversed through low-current spin injection. Also, the influence of magnetic field leakage from the storage layer and the reference layer has become more and more conspicuous as magnetoresistive elements have become smaller in size. In view of this, there is a demand for thinner layers. In a case where the respective layers are made thinner, a ferromagnetic layer is greatly affected by the roughness of the interface with the base layer and diffusion of the elements contained in the base layer. Therefore, there is a demand for a material and a film formation method that can reduce the roughness of the interface and the diffusion of the elements.